Planets In the Solar System

The wide variety of objects that exist in the Solar System fall into several categories. In recent years many of these categories have been found to be less clear-cut than once thought. This encyclopedia employs the following divisions:

Space debris of artificial origin that can be found in orbit around Earth.

Planetesimals, from which the planets were originally formed, are sub-planetary bodies that accreted during the first years of the Solar System and that no longer exist. The name is also sometimes used to refer to asteroids and comets in general, or to asteroids below 10 km in diameter.

Asteroids are objects smaller than planets that lie roughly within the orbit of Jupiter and are composed in significant part of non-volatile minerals. They are subdivided into asteroid groups and families based on their specific orbital characteristics.

Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners.

Trojan asteroids are located in either of Jupiter's L4 or L5 points, though the term is also sometimes used for asteroids in any other planetary Lagrange point as well.

Meteoroids are asteroids that range in size from roughly boulder sized to particles as small as dust.

Comets are composed largely of volatile ices and have highly eccentric orbits, generally having a perihelion within the orbit of the inner planets and an aphelion beyond Pluto. Short-period comets exist with apoapses closer than this, however, and old comets that have had most of their volatiles driven out by solar warming are often categorized as asteroids. Some comets with hyperbolic orbits may also originate outside the Solar System.

Centaurs are icy comet-like bodies that have less-eccentric orbits so that they remain in the region between Jupiter and Neptune.

Kuiper belt objects have orbits lying between 30 and 50 AU (astronomical units, an AU is approximately equal to the mean distance between Earth and Sun). This is thought to be the origin for short-period comets. Pluto is sometimes classified as a Kuiper belt object in addition to being a planet, and the Kuiper belt objects with Pluto-like orbits are called Plutinos. The remaining Kuiper belt objects are classified as Cubewanos in the main belt and scattered disk objects in the outer fringes.

Oort cloud objects, currently hypothetical, have orbits lying between 50,000 and 100,000 AU. This region is thought to be the origin of long-period comets.

The newly discovered object 90377 Sedna, with a highly elliptical orbit extending from about 76 to 928 AU, does not obviously fit in either category, although its discoverers argue that it should be considered a part of the Oort cloud.

Mosaic of Solar System planets except Pluto, including Earth's Moon (not to scale).

Jupiter constitutes most of the mass of the Solar System outside the Sun: 0.1% of the mass of the Solar System. In turn, Saturn constitutes most of the remaining mass, then Uranus and Neptune, then Earth and Venus (see also below).

Origin and evolution of the Solar System

The Solar System is believed to have formed from the Solar Nebula, the collapsing cloud of gas and dust which gave birth to the Sun. As it underwent gravitational collapse, the Solar Nebula would have collapsed into a disk, with the protosun accreting at the centre. As the protosun heated up, volatile substances were driven away from the central regions of the nebula - hence the formation of rocky planets closer to the sun and gas giants further out.

For many years, our own system was the only planetary system known, and so theories only had to explain one system to be plausible. The discovery in recent years of many external systems (see Exoplanet) has uncovered systems very different to our own, and theories of planetary system formation have had to be revised accordingly. In particular, many external systems contain a hot Jupiter - a planet comparable to or larger than Jupiter orbiting very close to the parent star, perhaps orbiting it in a matter of days. It has been hypothesised that while the giant planets in these systems formed in the same place as the gas giants in our system did, some sort of migration took place which resulted in the giant planet spiralling in towards the parent star. Any terrestrial planets which had previously existed would presumably either be destroyed or ejected from the system.

Galactic orbit of the Solar System

Estimates place the Solar System at between 25,000 and 28,000 light years from the galactic center. Its speed is about 220 kilometres per second, and it completes one revolution every 226 million years. At the galactic location of the Solar System, the escape velocity with regard to the gravity of the Milky Way is about 1000 km/s.

The Solar System appears to have a very unusual orbit. It is both extremely close to being circular, and at nearly the exact distance at which the orbital speed matches the speed of the compression waves that form the spiral arms. The Solar System appears to have remained between spiral arms for most of the existence of life on Earth. The radiation from supernovae in spiral arms could theoretically sterilize planetary surfaces, preventing the formation of large animal life on land. By remaining out of the spiral arms, Earth may be unusually free to form large animal life on its surface.

Discovery and exploration of the Solar System

Because of the geocentric perspective from which humans viewed the Solar System, its nature and structure were long misperceived. The apparent motions of Solar System objects as viewed from a moving Earth were believed to be their actual motions about a stationary Earth. In addition, many Solar System objects and phenomena are not directly sensible by humans without technical aids. Thus both conceptual and technological advances were required in order for the Solar System to be correctly understood.

The first and most fundamental of these advances was the Copernican Revolution, which adopted a heliocentric model for the motions of the planets. Indeed, the term "Solar System" itself derives from this perspective. But the most important consequences of this new perception came not from the central position of the Sun, but from the orbital position of the Earth, which suggested that the Earth was itself a planet. This was the first indication of the true nature of the planets. Also, the lack of perceptible stellar parallax despite the Earth's orbital motion indicated the extreme remoteness of the fixed stars, which prompted the speculation that they could be objects similar to the Sun, perhaps with planets of their own.

Since the start of the space age, a great deal of exploration has been performed by unmanned space missions that have been organized and executed by various space agencies. The first probe to land on another Solar System body was the Soviet Union's Luna 2 probe, which impacted on the Moon in 1959. Since then, increasingly distant planets have been reached, with probes landing on Venus in 1965, Mars in 1976, the asteroid 433 Eros in 2001, and Saturn's moon Titan in 2005. Spacecraft have also made close approaches to other planets: Mariner 10 passed Mercury in 1973.

The first probe to explore the outer planets was Pioneer 10, which flew by Jupiter in 1973. Pioneer 11 was the first to visit Saturn, in 1979. The Voyager probes performed a grand tour of the outer planets following their launch in 1977, with both probes passing Jupiter in 1979 and Saturn in 1980-1981. Voyager 2 then went on to make close approaches to Uranus in 1986 and Neptune in 1989. The Voyager probes are now far beyond Pluto's orbit, and astronomers anticipate that they will encounter the heliopause which defines the outer edge of the Solar System in the next few years.

Pluto remains the only planet not having been visited by a man-made spacecraft, though that will change with the launching of New Horizons by NASA in January 2006. It is scheduled to fly by Pluto in July 2015 and then make an extensive study of as many Kuiper Belt objects as it can.

Through these unmanned missions, we have been able to get close-up photographs of most of the planets and, in the case of landers, perform tests of their soils and atmospheres. Manned exploration, meanwhile, has only taken human beings as far as the Moon, in the Apollo program. The last manned landing on the Moon took place in 1972, but the recent discovery of ice in deep craters in the polar regions of the Moon has prompted speculation that mankind may return to the Moon in the next decade or so. Manned missions to Mars have been eagerly anticipated by generations of space enthusiasts, and it was hoped that the first manned interplanetary flights would take place in the 1980's, after the successful Apollo program. The United States now plans manned Lunar and Mars missions as part of the new Vision for Space Exploration.

The Solar System and other planetary systems

Until recently, the Solar System was the only known example of a planetary system, although it was widely believed that other comparable systems did exist. A number of such systems have now been detected, although the information available about them is very limited. See extrasolar planet for more information.

Attributes of major planets

* See Earth article for absolute values.** Soon after its discovery in 1930, Pluto was classified a planet by the International Astronomical Union. However, based on additional discoveries since that time, some astronomers have suggested reconsideration of that decision.

Attributes of selected minor planets

Some objects are intermediate in size between planets and the lumps of rock called asteroids. These mid-sized objects are now often called 'planetoids' or minor planets: most scientists consider them too small to be "true" planets, while a few scientists point out that these minor planets exhibit the same gravitational forces which affect major planets.

Just one planetoid, Ceres, lies in the inner reaches of the Solar System. All other planetoids occur at the fringe of our planetary system.

Other facts

The total surface area of the Solar System's objects that have solid surfaces and a diameter greater than 1 km is ~1.7×109km2 —about 11 times the area of the Earth's land masses. ([1] (http://uts.cc.utexas.edu/~gwk/images/SolSys/SolSys.html))

It has been suggested that the Sun may be part of a binary star system, with a distant companion named Nemesis. Nemesis was proposed to explain some timing regularities of the great extinctions of life on Earth. The hypothesis says that Nemesis creates periodical perturbations in the Oort cloud of comets surrounding the Solar System, causing a "comet shower". Some of them hit Earth, causing destruction of life. This hypothesis is no longer taken seriously by most scientists.

Edge of the Solar System

The point at which the Solar System ends and interstellar space begins is not precisely defined, since its outer boundaries are delineated by two separate forces: the solar wind and the Sun's gravity.

The charted regions of our Solar System exist within a highly tenuous "atmosphere" of solar wind; charged particles eminating from the Sun that expand outward in a great bubble to about 95 AU (three times the orbit of Pluto). The edge of this bubble is known as the termination shock; the point at which the solar wind collides with the opposing winds of the interstellar medium. Here the wind slows, condenses and becomes more turbulent, forming a great oval structure known as the heliosheath that extends outward for a further 40 AU at its stellar-windward side. The outer boundary of the sheath, the heliopause, is the point at which the solar wind finally terminates, and one enters the environment of interstellar space. Beyond the heliopause, at around 230 AU, lies the bow shock, a plasma "wake" left by the Sun as it travels through the Milky Way.

But even at this point, we could not be said to have left the Solar System, for the Sun's gravity will still hold sway even up to the Oort Cloud, the great mass of comets which surrounds our Solar System like a shell and extends from 50,000 to 100,000 AU (nearly a light year) beyond the Sun.

The Solar System in small scales

Scaling down the size of the Solar System makes it easier for students to grasp the relative distances.
The enormous ratio of interplanetary distances to planetary diameters makes constructing a scale model of the Solar System a challenging task. (For example, the distance between the Earth and the Sun is almost 12,000 times the diameter of the Earth.) Several places have built such models. See main article:Solar System model.